i am no metals expert, but from what i understand, forged items are stronger than billet, and billet is better than cast. think of it like this, a billet is a block of metal that you remove metal from to form a shape. a forging is a piece of metal that has been rough shaped by massive amounts of pressure. the process of forging makes the metal denser in the thin areas. an axe head for example is shaped by beating hot metal with a hammer, and then the eye for the handle is done the same way, by beating a tool through the piece. while i know there is alot more to it than just that, that is the simplest way i know to describe it.
i race off-road, baja and short course. the big trucks with 900 hp that weigh up to 7000#, mostly use forged wheels because they are a little bit stronger than billet. also the high dollar aluminum engine blocks start with a forging. cast is the cheapest, and is avoided when possible.